RFR: 8250887: Crash adjusting weak references during GC

[Erik Österlund]

Fixes an issue that SerialGC crashes due to its adjust closure incorrectly being applied to the nmethod keepalive
object.

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Commit messages:
 - 8250887: Crash adjusting weak references during GC

Changes: https://git.openjdk.java.net/loom/pull/15/files
 Webrev: https://webrevs.openjdk.java.net/loom/15/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8250887
  Stats: 18 lines in 7 files changed: 6 ins; 0 del; 12 mod
  Patch: https://git.openjdk.java.net/loom/pull/15.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/15/head:pull/15

PR: https://git.openjdk.java.net/loom/pull/15